This invention relates to differential amplifiers and, more particularly, to a bipolar differential amplifier integrated circuit and method for trimming the offset voltage thereof, the trimmed voltage of which is temperature independent.
Bipolar differential amplifier integrated circuits are well known. A typical trimmable bipolar differential amplifier includes a pair of transistors the emitters of which are differentially connected to a current source which provides a "tail" current to the transistors. The collectors of the two transistors may be coupled to a current mirror circuit which converts the differential input signal to a single-ended output signal as understood. The differential input signal is applied across the bases of the two transistors. The current mirror circuit may include resistive trimmable elements and is comprised of a diode coupled in series with collector of the first one of the pair of transistors and the first resistive element to ground reference potential and a third transistor whose base is connected to the collector of the first transistor and whose collector-emitter conduction path is coupled in series with the collector of the second transistor and second resistive element to the ground reference potential. U.S. Pat. No. 3,872,323 discloses a differential amplifier of the type discussed above.
Ideally, if all the elements of the differential amplifier are perfectly matched, a zero offset voltage is produced at the differential inputs, i.e., the bases of the two transistors as the differential amplifier is in a quiescent operating state. However, due to process tolerances in large volume production, there is almost always some mismatch of the elements. This mismatch produces a difference voltage across the bases of the two transistors as understood which is undesirable.
In the past the resistive elements have been trimmed using conventional trimming techniques to reduce the offset voltage to zero or as close to zero as possible.
A problem with most prior art differential amplifiers is that resistive trimming of the current mirror causes the semiconductor devices to be operated at different current densities. Therefore, although the offset voltage can be trimmed to zero volts at room temperature it has a temperature coefficient (TC) that is not zero. Hence, the offset voltage does not remain zero as the differential amplifier is operated over temperature.
Hence, a need exists for a differential amplifier and method for trimming the offset voltage thereof to substantially zero with the trimmed offset voltage being temperature independent.